In the past, scrap skin and leather have been discarded as having no useful purpose. The pieces of scrap were insufficient to utilize for leather goods of the like. People had attempted to grind and pulverize the skin/leather and mix it with resins so that commercial articles could be made therewith. Skin and leather breathes and because of this it has a comfortable feeling when worn as a garment or as footwear. Synthetics generally do not breath. The prior art incorporating skin/leather into the resin was unsatisfactory because the particles of skin/leather tended to clump or lump together. The failure of the particles to be uniform in distribution throughout the synthetic resin material created undesirable characteristics to the material molded therefrom.
Natural leathers have long been used for clothing and other articles. They wear well and are supple and tough as well as attractive in appearance. Unfortunately, natural skins and leathers are limited in supply and because of troublesome tanning processes, become quite expensive when used in a manufacture of articles of clothing, accessories or the like. Because of different shapes and sizes of the original animals, a special skill is needed to cut and make them into clothes, etc. and scrap material and waste is common.
Previous attempts have been made to pulverize the skin/leather into a powder. When used with a resin, the powder tends to cake.
Recently, various artificial and synthetic leathers have been developed as a substitute for natural leather. One typical example is a synthetic leather comprising a substrate of woven or knitted fabric and a sponge-like surface made of formed compositions such as vinyl chloride, polyamides or polyurethanes treated with the denatured polyamides, polyurethanes, or polyacrylic acid derivatives.
Another example is an artificial leather having a porous structure comprising a three-diminsionally entangled fiber member made of special fibers such as hollow, lotus-root-like fiber, a bundle of very fine fibers, and a binder mainly of an elastomer(polyurethane) and the like.
These synthetic or artificial leathers have characteristics very similar to those of natural leathers and have excellent insulating qualities and yet have excellent air and moisture permeability and water repellency. However, water absorption in these artificial and synthetic leathers occurs only through capillary action, and water absorption due to swelling of tissues does not occur in them. While air permeability is controlled in natural leathers as the porosity in the tissues is adjusted when the tissues swell, synthetic or artificial leathers do not have such an action.
Natural skins refer to untanned pelts. Natural leathers refer to leather tanned pelts. Tanned pelts require tanning treatment with tanning or chrome. Numerous attempts have been made to pulverize natural skin/leather into a powder and to mix it with synthetic resins to be molded into sheet using the resin as a binder. If natural skin or leather could be molded together with synthetic resin, in a proper manner, a number of advantages would be apparent. For one, the presence of natural skin or leather in the molded product would improve water and moisture absorption due to swelling and provide a molded product having an excellent feel without stickiness. Secondly, the presence of skin or leather powder in the molded product should provide suppleness and resillient toughness to the product not obtained in other resin molded products.
Thirdly, the presence of skin and leather powder should reduce static build-up and eliminate the static discharges and the discomfort caused thereby. It would also reduces the adherence of dust caused by static build-up.
Further, the heat insulating property of the resin molded products containing skin and leather powder should be greatly improved.
Difficulties have been experienced in pulverization, since longer fibers sometimes became entangled depending on the type of skin or leather (whether pelt of a pig or cow; a particular portion of a pelt, etc. or other properties such as whether the pelt is of a young animal or of an adult; or the type of tanning or storage conditions under which the pelts were treated).
Furthermore, problems have been encountered in going from a pilot plant to actual production.
Other problems have been noted and it was also found that due to forced drying and impact shearing, grains of the skin or leather powder got entangled and took on an appearance of unraveled cotton during storage. This powder eventually flocculated making it difficult to mix with synthetic resin.
Flocculated skin or leather powder will not ordinarily mix evenly in the synthetic resin due to the lack of lubricity. This making it impossible to give the product properties specific to leather and interferes with the good characteristics of the molded resin itself.
The presence of flocculated skin or leather powder further led to defectively high increases in resistance in the mixing cylinders, die tools used for extrusion or injection molding. Thus wear of the cylinder inner wall or damage to the dies or molding tools developed. When resistance in extrusion or injection was high, it was necessary to use larger molding machines and the molded products had deformations due to shrinkage, etc.
Although the problem of fluidity in molding with a calender roll might have been minimal, flocculated skin or leather contained in the synthetic resin would often agglomerate or cake, and the product would become uneven on the surface. The caked skin/leather powder would also cause cuts or abrasions in the surface of the resin material.
Further in the past, when mixed with a paint, the flocculated skin or leather powder became unevenly distributed in the coating layer causing bumps or scars in the coated surface. Additionally, the paint, when used in a spray gun, tended to clog the nozzle of the gun.
Other problems in the prior developments occurred when a film of less than 0.1 mm thickness was to be produced. The powder caused bumps in the film and weakened certain areas which eventually led to tearing of the film.
Further, in the prior art method, problems of uneven grain size distribution and longer fibers were encountered in pulverizing cow hides because of their tough collagen fiber. With pig skin, on the other hand, oil/fat content in the skin hampered the efficiency of pulverization.